Selective flash fusing



' Oct. 21, 1969 LE|GA ET AL SELECTIVE FLASH FUSING 2 Sheets-Sheet 1Filed Dec 2, 1966' ABSORBTIVITY (PERCENT) L WAVELENGTH (m FIG.

PULSE DURATlON (SE0) DEGREE OF SELECTI VITY INVENTORS RD 6. LEIGI ES F.GALLC Y A. WALDEF 16 4 f FIG. 2

,4 1;, A TTORNEV Oct. 21, 1969 Filed Dec. 2, 1966 A. G. LEIGA ET AL3,474,223

sELEcnviFLAsn'Fusme 2 Sheets-Sheet 2 ,4 /4 5 gg I6 V I 0 I v v w L DCPOWER R HIGH VOLTAGE SUPPLY P4 PULSE .24 TRIGGER CIRCUIT 20 INVENTORSALGIRD G. LEIG CHARLES F. GALL' BY ROY A. WALDE! A "OR/var United StatesPatent SELECTIVE FLASH FUSING Algird G. Leiga, Pittsford, Charles F.Gallo, Penfield, and Roy A. Walder, Rochester, N.Y., assignors to XeroxCorporation, Rochester, N.Y., a corporation of New York Filed Dec. 2,1966, Ser. No. 598,811 Int. Cl. H05!) 1/02 US. Cl. 219216 13 ClaimsABSTRACT OF THE DISCLOSURE Method and apparatus for achieving selectiveflash fusing of an electroscopic toner image by applyinga pulse ofradiation for a duration of time long compared to the time constant forheat loss from isolated background particles but short compared to thetime constant for heat loss from the toner image so that the net heatadded is sufficient to fuse only the image.

In general, the present invention relates to techniques of fusing andmore specifically to a technique for selectively flash fusing and theassociated apparatus.

In the process of xerography, for example, as disclosed in US. Patent2,297,691, issued Oct. 6, 1942, a xerographic plate comprising a layerof photoconductive insulating material on a conductive backing is givena uniform electrostatic charge over its surface and is then exposed tothe subject matter to be reproduced, usually by conventional projectiontechniques. This exposure discharges the plate areas in accordance withthe radiation intensity that reaches them and thereby creates anelectrostatic latent image on or in the photoconductive layer.Development of the latent image is effected by an electrostaticallycharged, finely divided material such as an electroscopic powder that isbrought into surface contact with the photoconductive layer and is heldthereon electrostatically in a xerographic powder image patterncorresponding to the electrostatic latent image. Thereafter, thedeveloped xerographic powder image is usually transferred to a supportsurface to which it usually is affixed.

Typical forms of electrostatic or electroscopic powder or tonercompositions used for developing are usually of a pigmented resin suchas disclosed in US. Patents Nos. 2,788,288, and 2,892,794 and US.Reissue 25,136.

A common method by which a powder image is fixed is by the process ofheat fusing, that is, by the application of heat in which case thepowder image or its support must be formed of a thermoresponsivematerial, such as a heat fusible resin, which flows without imagedistortion when heated and which coalesces and adheres to the surfacewhen cooled to ambient temperature.

In order to fuse resinous powder images, it is necessary to heat thepowder'and the paper to which it is to be fused to a relatively hightemperature. For given materials a temperature range exists in whichfusing will be produced. Below that temperature range the resinouspowder will not properly adhere to the support surface. If thetemperature is too high, there is a tendency for the support material todiscolor or scorch and in some cases for the toner to explode or bevaporized.

Various techniques have been developed for fusing in prior art. Amongthese are oven fusing, hot air fusing, radiant fusing, hot and coldpressure roll fixing and fusing, and flash fusing. Each of thesetechniques by itself has suffered from limitations and deficiencieswhich made them inapplicable for certain specific fusing jobs which arerequired in xerographic technology. In general, it has been difficult toachieve an entirely satisfactory design of heat fusers with regard toshort warm-up time, low electric current requirements, adequate heatinsulation 3,474,223 Patented Oct. 21, 1969 and uniform heatdistribution. Specifically, hot air and oven systems tend to be slow andinvolve high power consumption. Hot and cold pressure systems havepresented problems of offsetting, resolution degradation, poor fixingand limited quality.

Flash fusing has been desirable for some time since it is very efficientat slow or intermittent reproduction speeds but still suitable for highspeed copying. A major problem with flash fusing as known in the priorart has been that it was not selective. Since the term selective hasbeen used in various ways in connection with fusing processes in thepast, it should be clearly understood that it is herein referred to asthe preferential fusing of dense image areas leaving low density orbackground areas unfused. The undesirable, unfused background can thenbe wiped off or otherwise removed to yield a cleaner, more readablecopy. Thus, since in the past it has been believed that flash fusingcould not be selective fusing in the sense employed herein, this problemis one of the major problems toward which the present invention isdirected.

Accordingly, it is an object of this invention to provide a new anduseful, highly effective, and elficient selective flash fusing techniqueand apparatus which overcome the deficiency of the prior art asdescribed above.

It is a further object of this invention to achieve both flash andselective fusing by the same process.

It is an additional object of this invention to achieve a high classselective fixing of toner images with minimum total energy in comparisonwith existing selective fusing systems.

It is also an object of this invention to avoid producing documentwarpage by the flash process.

Yet another object of this invention is to eliminate long warm-up timerequired prior to fusing.

An additional object of the present invention is to provide forselective fusing at high speeds.

Other objects and fuller understanding of the invention may be had byreferring to the following description and claims taken in con unctionwith the accompanying drawings.

The present invention overcomes the deficiencies of the prior art andachieves its objectives by tailoring the pulse duration and energydensity delivered to the toner particles so as to provide selectiveflash fusing. In order to facilitate understanding of this invention,reference will now be made to the appended drawings of a preferredembodiment of the present invention. The drawings should not beconstrued as limiting the invention but are exemplary only.

In the drawings;

FIGURE 1 is a representation of the absorptivity of paper and toner as afunction of wavelength.

FIGURE 2 is a representation of the degree of selectivity for givenpulse durations in flash fusing.

FIGURE 3 is a schematic representation of the heat loss phenomenoninvolved in selective flash fusing.

FIGURE 4 is a schematic representation of the apparatus of the presentinvention.

While it is not intended to limit the invention to any specific theoryof operation, it is presently believed that the following analysisexplains the relevant criteria in tailoring the pulse duration inselective flash fusing to achieve the desired objectives of the presentinvention.

When toner particles 14 are placed upon a sheet of material 16 to forman image in a xerographic or electrostatic printing process, a majorityof the toner particles are accumulated in the area of the image pattern.Scattered about in areas not intended to make up a part of the image,are single isolated toner particles or small clumps of such particleswhich are referred to as the background. This light background aroundthe image area is not to be fused. In selective fusing it is desired tofuse only the dense image areas.

It is known that the selectivity of radiant fusing is enhanced if thespectral output of the light source is such that the wavelengthabsorptivity of the tone 14 is a maximum and if simultaneously thewavelength absorp tivity of the paper 16 is a minimum. It may be seen inFIGURE 1' that this is in fact the case for the selectivity orabsorptivity of the paper in and around the area of the visible regionof light. The spectral output of xenon flash lamps is particularlydesirable for selective fusings although other lamps may be utilized. Itshould be understood that by achieving the above spectral control interms of absorptivity for a given wavelength output by the lamp, theeffect is to heat the toner and not to heat the paper. If the paper wereheated to sufficient temperature, all of the toner including thebackground would become fused to it. It is this type of situation whichthe present invention seeks to avoid. Among the conditions andparameters utilized in the present invention to obtain selective flashfusing is the control of the spectral output to optimize the transfer ofenergy, thus obtaining selective flash fusing. However, the control ofspectral output alone has not been found to be sufficient to provideselective flash fusing.

A prior art model for the toner fusing phenomenon is that of a singlelayer of spherical toner particles 14 whose mean separation determinesthe optical density of the image or background. However, in such a modelthe energy absorbed per unit area is determined merely by the meanseparation of the toner particles. Thus, while the energy absorbed perunit area of the paper surface will increase as the surface density ofthe toner particles increases, it is also clear from the model shown inFIG- URE 3 that the energy absorbed per toner particle remainsessentially the same and is independent of whether a toner particle iscompletely isolated or whether it is surrounded by other toner particlesresting on the paper surface. Since the energy absorbed per tonerparticle determines toner temperature, if the effect of heat losses fromthe toner particles is ignored, such a model would lead to theconclusion that a uniform toner temperature is achieved throughout thesheet independent of optical density. Based upon such reasoning andunsuccessful experiment efforts, the prior art concluded that flashfusing could not be made selective.

However, when the heat loss from each toner particle 14 is considered asshown in FIGURE 3, selectivity may be produced by using the tailored,pulsed output of a xenon flash lamp 12 with reflecting element whichproduces an output which is partially in the visible region andsatisfies the spectral requirements for achieving selectively asindicated in FIGURE 1. It has been discovered that the flash duration isa critical parameter in achieving selectivity. If the flash duration isless than approximately a millisecond as shown in FIGURE 2, little or noselectivity is achieved. If the flash duration is greater than amillisecond, selective flash fusing is achieved. A prime reason for thisselectivity with longer pulses is that the net rate of heat loss by asingle toner particle 14 is decreased by the presence of adjacent tonerparticles 14 as shown in FIGURE 3. Since any adjacent toner particlesdecrease the effective surface area of a toner particle available forthe heat loss by conduction, convention, or radiation, any heat flowbetween adjacent toner particles essentially conserves heat in the tonersystem and thus clumps of toner particles get hotter initially and stayhotter longer.

The reason for the above described behavior can be understood in termsof the following discussion based upon the time constants of heat lossfrom the toner particles, where the time constant (T,,) is the timerequired for the temperature of the toner particles (n) to decrease tosome arbitrary fraction, such as, for example, l/e of its initial value,where e is the base of the natural logarithms and has a value of 2.71828As noted above and shown in FIGURE 3, the rate of heat loss from anisolated toner particle is greater than the rate of heat loss from atoner particle in a clump of toner particles. In different terminology,the time constant (T for heat loss from an isolated or background tonerparticle is less than the time constant (T for heat loss from a tonerparticle in a clump forming a part of the image pattern. That is, T T Ifthe optical pulse duration (T is long compared to the time constant (Tof the background but short compared to the time constant (T of theimage areas, then selectivity of fusing will be achieved. Schematically,the duration (T of the optical pulse should be for selective flashfusing.

It has been determined by experimentation that the time constant for therate of heat loss by a single isolated toner particle is on the order ofa millisecond; thus, if the flash duration is less than one millisecond,a single isolated toner particle gets and remains as hot as a tonerparticle which is adjacent to other toner particles. If, however, theflash duration is longer than a millisecond with a measured inputintensity, an isolated toner particle will not become very hot even whenclumps of toner fuse because an appreciable amount of heat is allowed toleak away by the large surface area of that single toner particle inthat time interval. However, the toner particles which make up a clumpor form part of the image retain the heat better and stay hotter longer.Thus, the image toner is selectively fused while the other isolated andrandomly scattered particles are not. In an idealized manner thisexplanation accounts for selective fusing of toner particles. However,it should be realized that in actual practice the dense image area maycontain two or more layers of toner particles and that the tonerparticles may consist optically of two phases: a carbon particle actingmuch like a black body and a toner polymer which is relativelytransparent. Also, in practice the possibility of multiple reflectionsand absorptions may be taken into account. It should be noted that inpractice the spectral characteristics of the system, the time of pulseduration and the energy density received at the fusing surface arevariables which may be adjusted to achieve the desired results. Therelevant considerations concerning spectral characteristics and pulseduration have been discussed above. In general, the required energydensities and the intensity of the flash required to fuse tonerscommercially available are well known in the prior art for conventionalsystems. In any case, with the pulse duration established as describedabove and held constant, the intensity of the flash may be adjusted in agiven system until the dense image areas are fused as desired. Asindicated in FIGURE 2, if it is desired to fuse all toner including thebackground on a document, it is more efficient to employ flashes oftherequired energy with durations less than one milisecond so that thetoner does not have sufficient time to lose heat. If the flash durationof fusing energy is too short, however, the instantaneous tonertemperature will rise so quickly as to cause vaporization anddecomposition. If one wishes to achieve selective flash fusing, theduration of the flash should be greater than approximately onemillisecond and generally, in the range of 10- to 10' seconds. It shouldbe noted from the above discussion that the selective flash fusingphenomenon is highly dependent on the pulse duration. Also, for flashfusing the rise of time of the pulse should be as gradual as possible tohelp to avoid excessive toner temperatures.

For the sake of clarity for the disclosure and simplicity of explanationthe invention has been herein described above in terms of the theory ofoperation as presently understood although it is to be clearlyunderstood that the theory is illustrative only and is not intended tobe interpreted in the limitation of the scope of the invention.

As indicated by the above analysis, the production of selective flashfusing involves the transfer of the proper amount of energy with theproper pulse duration to transfer a sufficiently high intensity ofradiation while simultaneously avoiding any deleterious effects to thesubstrate on which the toner particles are to be fused.

A preferred embodiment of the present invention for achieving thistailored pulse duration is shown in FIG- URE 4. A DC power supplyindicated at 20 is connected across a capacitor 24 which may be groundedon one side at 22. This storage capacitor 24 typically has a value onthe order of 150 microfarads and serves to build up from 2,000 to 5,000volts maximum in stored condition for use when the flash lamp 28 is tobe pulsed. The storage condenser 24 is connected to the flash lamp 28through a variable inductor 26 which is typically in range of 150microhenrys to 3 millihenrys and determines the pulse duration producedby the flash lamp.

The flash lamp 28 consists of an envelope containing xenon gas and apair of electrodes at each end which are not electrically connected toeach other. Surrounding the glass envelope of the flash lamp 28 is acoil 30 which is connected to a high voltage pulse trigger circuit 34.Upon pulsing trigger circuit 34 a surge of current at approximately20,000 to 30,000 volts passes through the coil 30. The flow of currentthrough coil 30 couples with the electrodes of flash lamp 28 causing agas breakdown and pulsing of the flash lamp 28 which results in a flashof suitable duration as determined by inductance of 26.

While reference has been made throughout to such elements as the xenontube as the preferred embodiment, it is obvious that any other suitableflash tube may be used. Any suitable power supply and pulser may be usedand any other equivalent electrical circuitry may be utilized to producea tube pulse of suitable duration for selective flash fusing. In asimilar manner, alternating significantly the geometry of the light pathor the total energy transferred may suggest alternations in the pulsedurations and energy transfers required to bring about selective flashfusing. Toners other than the typical electroscopic toner compositionsreferred to above may be utilized. Specific modifications of papers andtoners consisting of different chemical composition than commonlyemployed within the term toner may also be employed with suitablealternations in the tailoring of the pulse duration in accord with theprinciples set forth in this invention. In operation the bringing of asheet with toner into position may automatically trigger the switchresulting in appropriate pulse of the flash lamp source.

As described above in the discussion of the principles of thisinvention, time durations greater than approximately 1 millisecond areutilized to achieve selective flash fusing. So long as a suitable energydensity in a given period of time is applied to produce the optimumselected flash fusing, which consists of not fusing the background andnot leaving the image unfused, the apparatus will be suflicient toaccomplish the purposes of the present invention. Longer pulse durationsthan the one millisecond pulse duration expressed as the duration of thepreferred embodiment in this invention may be achieved by the use oflarge inductors, by storing energy at low voltages and high capacity,the use of special flash lamps, and special pulse forming circuitdesigns.

Using the above apparatus and applying the above theory, the selectivefusion of line copy toner images has been accomplished with a singleflash of a six-inch long cylindrical design flash tube which was placedalong the focal axis of an aluminum sheet reflecting, paraboliccylinder. With the copy resting on an aluminum plate 6 /2 inches below,the light input energy was determined to be approximately 600 jouleswith a pulseduration of three milliseconds. Thus, in operationa pulse ofsuitable energy input is created by a flash lamp having the tailoredtime or pulse duration suitable for the particular toner and paper anddistance configurations. This tailored pulse is utilized to produceselective flash fusing.

Although a specific preferred embodiment of the present invention hasbeen described in the detailed description, the description is notintended to limit the invention to the particular forms or embodimentsdisclosed herein since they are to be recognized as illustrative ratherthan restrictive. It will be obvious to those skilled in the art thatthe invention is not so limited. The invention is declared to cover allchanges and modifications of the specific examples of the inventionherein disclosed for purposes of illustration which do not constitutedepartures from the spirit and scope of the invention.

What is claimed is:

1. A method of achieving selective fusing of an electroscopic tonerimage by an optical pulse of suitable intensity onto a substrate forsupporting said image while leaving the isolated background particles inan unfused condition comprising:

(1) pulsing a flash lamp,

(2) applying said optical pulse for a duration of time long compared tothe time constant for heat loss from said isolated background particlesbut short compared to the time constant for heat loss from saidelectroscopic toner image, said pulse being of a suflicient intensity sothat the net heat added to said toner image is suflicient to melt saidtoner but the net heat added to said isolated background particles isinsuflicient to melt them.

2. The method of claim 1 wherein said duration of time is not less thanone millisecond.

3. The method of claim 2 wherein said duration of time is between 10*and 10- seconds.

4. The method of claim 3 wherein applying said or tical pulse producesan input energy of approximately 600 joules over a pulse duration ofapproximately three milliseconds.

5. A device for use in a xerographic apparatus to selectively fuse anelectroscopic toner image by an optical pulse of suitable intensity ontoa substrate for supporting said image while leaving the isolatedbackground particles in an unfused condition comprising:

(1) a flash lamp, and

(2) means to produce a pulse of light of said suitable intensity for aduration of time long compared to the time constant for heat loss fromsaid isolated background particles but short compared to the timeconstant for heat loss from said electroscopic toner image, said pulseintensity being such that the net heat added to said toner image issuflicient to melt said toner but the not heat added to said isolatedbackground particles is insuflicient to melt them.

6. The device of claim 5 wherein said means to produce a pulse of lightof said suitable intensity for a duration of time long compared to thetime constant for heat loss from said isolated background particles butshort compared to the time constant for heat loss from saidelectroscopic toner image comprises:

(a) means to energize said flash lamp, and

(b) means to apply an increasing amount of energy mput to said tonerparticles for a duration of time of not less than one millisecond.

7. The device of claim 6 wherein said duration of time is between 10-and 10- seconds.

8. A device for use in a xerographic apparatus to selectively fuse anelectroscopic toner image by an optical pulse of suitable intensity ontoa substrate for supportmg said image while leaving the isolatedbackground particles in an unfused condition comprising:

(1) a flash lamp,

(2) electrical means to energize said flash lamp,

(3) means to supply electrical power to said flash lamp to produceillumination of sufliicent intensity to cause fusing of toner particles,and

(4) electrical control means to produce a pulse of sufficient intensitylight for a duration of time long compared to the time constant for heatloss from said isolated background particles but short compared to timeconstant for heat loss from said electroscopic toner image, said pulseintensity being such that the net heat added to said toner image issufficient to melt said toner but the net heat added to said isolatedbackground particles is insuflicient to melt them.

9. The device of claim 8 wherein said duration of time is not less thanone millisecond.

10. The device of claim 9 wherein said duration of time is between 10*and 10 seconds.

11. A device for use in a xerographic apparatus to selectively fuse anelectroscopic toner image by an optical pulse of suitable intensity ontoa substrate for supporting said image while leaving the isolatedbackground particles in an unfused condition comprising:

(1) a flash lamp, and

(2) means to pulse said flash lamp at said suitable intensity for apredetermined duration of time such that the rate of heat transfer tosaid electroscopic toner during said pulse is less than the rate of heatloss by said isolated background particles but greater than the rate ofheat loss from said electroscopic toner image areas, said pulseintensity being such that the net heat added to said toner image issufficient to melt said toner but the net heat added to said isolatedbackground particles is insuflicient to melt them.

12. The device of claim 11 wherein said means to pulse said flash lampat said suitable intensity for a predetermined duration of time suchthat the rate of heat transfer to said electroscopic toner during saidpulse is less than the rate of heat loss by said isolated backgroundparticles but greater than the I rate of heat loss from saidelectroscopic toner image areas comprises:

(1) means to energize said flash lamp, and

(2) means to apply an increasing amount of energy input to said tonerparticles for a period of not less than one millisecond.

13. A device for use in a Xerographic apparatus to selectively fuse anelectroscopic toner image by an optical pulse of suitable intensity ontoa substrate for supporting said image while leaving the isolatedbackground particles in an unfused condition comprising:

(1) a flash lamp,

(2) electrical means to energize said flash lamp,

(3) means to supply electrical power to said flash lamp to produceillumination of sufficient intensity to cause fusing of toner particles,and

(4) electrical control means to produce a pulse of said suflicientintensity light having a duration of between 10- and 10- seconds,whereby said image will be fused w-hile said background will remainunfused.

References Cited UNITED STATES PATENTS 2,807,703 9/1957 Roshon 219-3'88x 2,844,733 7/1958 Miller et al 25-o 3,163,755 12/1964 Kotz et a1.250L-65 3,187,162 6/1965 Hojo et al 219-3ss JOSEPH V. TRUHE, PrimaryExaminer C. L. ALBRI'ITON, Assistant Examiner US. Cl. X.R. 250-65

